Seth

Seth is unlocked when you get soy milk and chocolate milk on the same run.

Achievements
Blue Yarn - Complete the Boss Rush. Milk Baby - Defeat Mom's Heart or It Lives on Hard. Tiny Horn - Defeat Satan. Seth's Hat - Defeat Isaac. Strawberry Milk - Defeat The Lamb. Eternal Flame - Defeat ???.
 * Blue Yarn Achievement.png
 * Milk Baby Achievement.png
 * Tiny Horn Achievement.png
 * Seth's Hat.png
 * Strawberry Milk Achievement.png
 * Eternal Flame Achievement.png

Trivia

 * Seth in the bible was the third son of Adam and Eve.